In the production of such circuit elements, workpieces in the form of semiconductor wafers coated with a photosensitive layer are illuminated through a mask by an appropriate optical system, usually with substantial image-scale reduction, so that subsequent development and chemical treatment may establish an array of conductive and nonconductive areas on the wafer surface.
In many cases, a given wafer must be subjected to several such exposures with intervening processing requiring removal from its support. Thus, correlation or cross-matching of the wafer positions during all exposure operations is essential. Such precise positioning in the exposure station requires the photoelectric detection of very fine indexing marks on the wafer surface by highly sensitive optoelectronic means whose field capture is generally extremely limited so that the wafer must arrive at that exposure station with a preliminary orientation designed to ensure that the indexing marks will lie within the window of detection of the adjustment means. This prealignment is advantageously done in a prepositioning station at the same time that another wafer undergoes illumination in the associated exposure station. The use of the same indexing marks in the prepositioning station for prealignment purposes, however, would require a duplication of the high-precision photoelectric detection equipment and would be economically unjustified inasmuch as the subsequent transfer to the exposure station introduces some unavoidable departures from the pre-established orientation of the semiconductive wafer.
Conventionally, therefore, the preadjustment is effected by less sensitive photoelectric means designed to detect a distinctive configuration such as a flat edge portion on the outer contour of the wafer; see, for example, U.S. Pat. No. 3,930,684. Since, however, the location of the indexing marks bears a definite relationship only with circuit array formed or to be formed but not with the periphery of the wafer, such a system is not entirely satisfactory in situations requiring cross-matching between successively produced patterns.